Layers having various functions have hitherto been formed in optical articles, electronic and electric articles, and articles made of magnetic materials. Further, various protective films are optionally provided for protecting these layers. High film strength and high adhesion to the layers having various functions are generally required of the protective film.
For example, the so-called "plasma displays" which utilize a luminescent phenomenon accompanying discharge is classified, according to the structure of an electrode (composed mainly of ITO) used, roughly into a direct current type plasma display wherein a metallic electrode is exposed on a discharge space and an alternating current type plasma display wherein a metallic electrode is covered with a dielectric layer. In the case of application of the plasma display to color televisions which are thin and have a large image plane, the alternating current type plasma display which has a memory function and can cope with an increase in size is suitable, and, in this case, a protective layer (a film composed mainly of magnesium oxide) is formed on the surface of the dielectric layer.
On the other hand, an antireflection layer (formed of, e.g., silicon dioxide or titanium dioxide) is formed on the surface of displays used in computers, image display equipment and the like from the viewpoint of preventing the reflection of external light to improve the visibility of the image on the display.
The so-called "functional films" are mainly formed of a metal oxide. Specifically, magnesium oxide or the like is used in a protective layer or the like for electrode members composed of ITO, tin oxide is used for various electronic and electric articles, iron oxide is used for articles made of magnetic materials, and silicon dioxide, titanium dioxide and the like are used for optical members. When these functional films are thin, they have hitherto been formed by EB vapor deposition, sputtering, CVD or the like, while when they are thick, a method has been used which comprises spray-coating a precursor of a metal oxide on a substrate to form a thick coating and baking the coating to convert the precursor to a metallic oxide (see, for example, Japanese Patent Publication Nos. 42579/1985, 59221/1988, and 13983/1982). Further, regarding the coating method, there is a method wherein a fine powder of a metal oxide is dispersed in a liquid binder which is converted to an oxide upon baking to form a functional film (Japanese Patent Laid-Open No. 283020/1994).
In the above coating methods, for example, regarding a MgO paste for use in the formation of a MgO film, by printing, as a protective layer for an alternating current type plasma display panel, it is known that the protective layer is poor in sputtering resistance unless uniform fine particles of MgO having a diameter of 0.03 to 0.3 .mu.m are contained and the fine particles of MgO are homogeneously dispersed in a binder.
Among the above methods, EB vapor deposition and methods using a vacuum process, such as sputtering and CVD, have low productivity and, hence, lead to a high cost. In addition, it is very difficult to house a large deposited product, such as a plasma display, in a vacuum chamber. Therefore, when the size of an image plane should be increased, the above methods are disadvantageous in cost of equipment and productivity.
The coating method has been extensively and intensively studied in the art because it is simple. The studies, however, provided no satisfactory results. The reason for this is as follows. For pastes with functional particles incorporated therein, which have been commercially available, the functional particles agglomerate, making it difficult for the functional particles to exist in a homogeneously dispersed state in the paste. This in turn makes it difficult to form a highly uniform protective layer, posing problems including that the conventional heat treatment process offers no satisfactory film strength, adhesion and other properties and cracking occurs in the film during heat decomposition. Therefore, the use of the above paste with functional particles incorporated therein in the coating on the surface of a display imparts no necessary properties to the resultant panel.
In addition, the diameter of magnesium oxide particles incorporated into the paste is large, and, the viscosity of the paste per se is also large, making it difficult to reduce the thickness of the protective layer. Therefore, even though the discharge initiation voltage and the driving voltage are minimized, problems of large thickness and poor transparency remain unsolved.
In order to solve the above problems, the present inventors have already proposed the use of a sol-gel process wherein a metallic compound having a hydrolyzable reaction site is hydrolyzed to prepare a metal hydroxide sol, the sol is coated on a substrate, and the coating is baked to form a film of a corresponding metal oxide film as a protective layer, thus enabling the thickness of the protective layer to be reduced (Japanese Patent Application No. 271827/1994).
In this process, however, some metal hydroxides formed upon hydrolysis are likely to precipitate. In particular, for example, in the case of a magnesium compound, magnesium hydroxide formed by hydrolysis instantaneously precipitates as particles and settles. When a coating composition with the magnesium hydroxide particles precipitated therein is coated on a substrate to form a coating which is then gelled by the conventional baking process (a baking process wherein no high temperature is used) to form a functional film, the film is formed on the substrate with the magnesium oxide particles stacked on top of one another. Therefore, the density of the film is so low that, in some cases, no satisfactory film strength can be provided. Further, the formation of a thin film using such a coating composition cannot provide a dense film, resulting in no satisfactory function as a protective layer.
For example, magnesium dimethoxide, Mg(OCH.sub.3).sub.2, frequently used as an alkoxide of magnesium in the sol-gel reaction is a powdery substance wherein methoxy groups have associated with one another. This substance, when used alone, is very difficult to be dissolved or dispersed in an organic solvent, making it difficult to prepare a stable sol solution.
On the other hand, a method wherein a solution of an organomagnesium in an organic solvent is coated on a substrate to form a coating which is then heat-decomposed to form a colorless, transparent magnesium oxide film is known from Japanese Patent Laid-Open No. 123657/1980. In this method for forming a magnesium oxide film, however, after the coating of the solution containing an organomagnesium on a substrate, hydrolysis proceeds from around the surface of the coating, resulting in successive precipitation of magnesium hydroxide particles. Therefore, the resultant magnesium oxide film has poor adhesion to the substrate.
Further, in the method, for forming a magnesium oxide film, described in Japanese Patent Laid-Open No. 123657/1980, the hydrolysis occurs mainly after coating of the solution containing an organomagnesium on a substrate. Therefore, there is a high possibility that organic materials produced as by-products in the hydrolysis, that is, impurities, are present within the magnesium hydroxide particles. In the course of the production of a magnesium oxide film, it is difficult to remove the impurities by scattering from the coating.
Further, due to the nature of the method, for forming a magnesium oxide film, described in Japanese Patent Laid-Open No. 123657/1980 wherein the hydrolysis occurs mainly after coating of the solution containing an organomagnesium on a substrate, it is difficult to control the particle diameter of precipitated magnesium hydroxide as desired. Consequently, the formed magnesium oxide film is likely to be opaque and, hence, unsatisfactory in transparency.